The invention relates generally to non-contact gauging systems employing a plurality of gauging sensors directed at predetermined points on a workpiece to be gauged. More specifically, the invention concerns calibration of one or more non-contact sensors with respect to an external coordinate frame of reference.
In the manufacture of parts and assemblies by processes such as stamping, extrusion, drawing and casting from materials such as sheet or bulk metal, plastic, film, rubber and the like, it is often required that accurate dimensional measurements be performed either on-line as the manufacturing process operates upon the part or assembly or off-line upon samples of gauged workpieces temporarily removed from the manufacturing production line. In order to obtain economies of manufacturing and produce high quality parts, it is necessary that measurements be made to ensure that dimensional tolerances are maintained. Such measurements must be made rapidly at many points on each produced part or assembly and in such a manner that the part or assembly itself is not touched or otherwise affected substantially by the measuring process itself. Measurements are often required to be performed which determine the position in three dimensional space of certain identified points or features which exist on the manufactured part or assembly being gauged. Such points or features typically include edges, corners, holes, designated portions of a surface, studs and intersections of surfaces. Special and general purpose computers exist which are capable of determining such measurements from image data produced by sufficiently intelligent sensor-illuminator systems.
One such sensor-illuminator system which operates on the triangulation principle to generate image data usable by a machine vision computer is disclosed in U.S. Pat. No. 4,645,348--Dewar, et al., assigned to the same assignee as the instant invention. As disclosed in the U.S. Pat. No. 4,645,348--Dewar, et al. the gauging sensor is internally calibrated with respect to the sensor's own coordinate system by use of so-called "rectification" tables. The contents of the rectification table give the coordinates in space relative to the coordinate system of the sensor of each point which reflects light of predetermined quality back to each corresponding picture element of "pixel" of the gauging sensor's light receptive sensing system.
The next step in calibrating a sensor such as that disclosed in the Dewar, et al. patent is to establish a point of reference external to the sensor for the measurements effected through use of the rectification table. In known measuring systems, such sensors are calibrated to some type of master or "golden" part. However, such calibration to a master part results in less than optimum measurement system calibration in the case of large work pieces, such as large portions of automotive vehicles. Where the part is very large, the corresponding master part is likewise large and prone to distortion when moved from one location to the next. Therefore, when moving a large master part to a gauging fixture, the part distorts and the calibration error will exceed acceptable levels.
An additional problem with prior calibration approaches is the necessity for very accurately determining the precise location in the external reference coordinate system of each sensor to be calibrated. Only in this way in the prior art could the sensor data be accurately related to the external point of reference. Such precise alignment of a plurality of sensors in a relatively large gauging station area has proven very difficult, if not infeasible.
Additionally, the user of the non-contact gauging system may desire to measure the same parts with a coordinate measuring machine which requires moving the part and mounting it in a different fixture. Because the part may be mounted differently in each fixture, the measurements will not be consistent.
Manufacturing operations are quickly moving into the era of using a common reference coordinate system from the design of the workpiece in a CAD/CAM environment and performing all measurements and manufacturing operations at each manufacturing station relative to a common coordinate system.
Therefore, there is seen to be a need for the capability of calibrating each sensor of a multi-sensor gauging array to a common external coordinate system rather than to the internal coordinate system of each individual sensor.